Electropolishing metallic articles



Nov. 29, 1955 H. STRQBEL 2,725,353

ELECTROPOLISHING METALLIC ARTICLES Filed July 1950 5 Sheets-Sheet 1 T 1 o E i I I 3 5 |E I1 2 i 9 (SIM E9 2} EQ I d I 2 9 2 5 N 3 z 3' k k I 0: ll I l 2 i i i S 72 I g Q g N\ a I 3 5 i :1 lNl/ENTOR --hiRSTROBEL ATTORNEY Nov. 29, 1955 H. R. STROBEL ELECTROPOLISHING METALLIC ARTICLES 5 Sheets-Sheet 2 Filed July 21, 1950 O O O O O- o o o lNl/ENTOR H. R. S TROBEL A T TORNEV NOV. 29, 1955 STROBEL 2,725,353

ELECTROPOLISHING METALLIC ARTICLES Filed July 21, 1950 5 Sheets-Sheet 3 FIG. 4

INVEN 70R hf R. STROBEL A TTORNEV 1955 H. R. STROBEL ELECTROPOLISHING METALLIC ARTICLES 5 Sheets-Sheet 4 Filed July 21, 1950 uws/v TOR H. R. S TROML ATTORNEY Nov. 29, 1955 H. R. STROBEL 2,725,353

ELECTROPOLISHING METALLIC ARTICLES Filed July 21, 1950 5 Sheets-Sheet s nl- W: H l

FIG. 7

INVENTOR /-I. R. STROBEL AT'I'URNEV United States Patent ELECTROPOLISHING METALLIC ARTICLES Herman R. Strobel, Fullerton, Md., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application July 21, 1950, Serial No. 175,124

6 Claims. (Cl. 204-32) This invention relates to electropolishing metallic articles, and more particularly to electropolishing metallic filaments, such as tapes, wires and the like.

Certain types of communication systems employ coaxial cable units of the type in which an outer tubular conductor made from a metallic tape is supported concentrically on a central, wire-like, conductor by spaced insulating discs, or the like. In the manufacture of such a coaxial unit, it is important that the inner surface of the tape and the surface of the central conductor used in making it be free of ridges, splinters, bumps, flakes and other projections in order for the electrical characteristics of the unit to be uniform throughout the length of the unit. In the past, this result has been difiicult to achieve. Other industries also require metallic articles having smooth, polished surfaces.

An object of the invention is to provide new and improved methods of electropolishing metallic articles.

A further object of the invention is to provide new and improved methods of electropolishing metallic filaments, such as tapes, wires and the like.

Another object of the invention is to provide new and improved methods of removing splinters, metallic flakes, ridges and other projections from the copper wires and tapes used in making coaxial cable units.

A method illustrating certain features of the invention may include immersing a copper article to be polished in an aqueous electrolyte including copper nitrate and sulphuric acid, and making the article an anode, whereby projections on the surface of the article are dissolved and the surface of the article is passivated.

A complete understanding of the invention may be obtained from the following detailed description of methods forming specific embodiments thereof, when read in conjunction with the appended drawings, in which:

Fig. 1 is a front elevation of a portion of an apparatus for effecting one method embodying the invention;

Fig. 2 is a corresponding view of another portion of the apparatus continued from the right hand end of Fig. 1;

Fig. 3 is an enlarged, partially sectional view of a portion of the apparatus shown in Fig. 1; I

Fig. 4 is an enlarged, vertical section taken along line 44 of Fig. 3; v

Fig. 5 is a front elevation .of a portion of another apparatus for effecting a method forming one embodiment of the invention;

Fig. 6 is a corresponding view of another portion of the apparatus continued from the right hand end of Fig. 5

Fig. 7 is an enlarged, top plan view of a portion of the apparatus shown in Fig. 5;

Fig. 8 is a vertical section taken along line 88 of Fig. 7, and

Fig. 9 is a view of a portion of one type of tape which may be treated by methods and baths embodying the invention.

-Referring now in detail to the drawings, a capstan 8 (Fig. 1) advances a filamentary copper conductor 10 of substantial length to be polished continuously through a 2,725,353 Patented Nov. 29, 1955 cleaning bath 12, rinsing bath 14, a cathode device 16 immersed in an electropolishing bath 18, a rinsing bath 20, an alkaline neutralizing bath 22 and a rinsing bath 24. The conductor 16) is advanced by the capstan 8 from a suitable supply reel (not shown) to a takeup reel 25. A rotary contact sheave 34 contacts the conductor 10 at a point in advance of the entrance thereof into the electropolishing bath 18, and the sheave 34 is connected to the positive side of a source of D. C. potential (not shown), the negative side of which is connected to the cathode device 16. The bath 12 is an alkaline bath, and removes grease and foreign substances from the surface of the conductor as it is advanced therethrough. The rinsing bath 14 is water, which rinses any alkali on the conductor therefrom.

The cathode device 16 includes hollow, plastic cylinders 39 and 40 (Figs. 3 and 4), around which smooth,

endless, stainless steel or copper sheet cathodes 42-42 are guided and continuously advanced. The cylinders are composed of insulating material, such as, for example, the phenol-formaldehyde condensation product Bakelite, and are provided with a series of perforations 4141 and extending therethrough. Cathode rolls 4444 advance and contact the sheet cathodes 4242, and guide rolls 46-46 guide the cathodes around the cylinders, The cathodes 42-42 are advanced continuously and scrapers 48-48 scrape copper deposited on the cathodes from the smooth surfaces thereof.

The area of the perforations in the cylinder 39 with respect to the surface area of the portion of the conductor 10 coextensive therewith is sufficiently small to limit the flow of current between the conductor and the sheet cathode on the cylinder 39 to an amount less than that at which passivation of the surface of the conductor occurs. The area of the perforations per unit of length of the cylinders 39 and 40 gradually increases from the entrance of the cylinder 39 to a point on the cylinder 40 may be termed the projection dissolving portion.

The composition of the electropolishing bath 18 should be from about 18% to about 30% by weight copper nitrate, from-about 6% to about 2% by weight sulphuric acid, and the balance water. The sulphuric acid increases the conductivity of the electropolishing bath, but the concentration thereof is sufiiciently low to prevent precipitating copper sulphate out of the solution, and the concentration of the sulphuric acid within the above-listed range is inverse with respect to the copper nitrate in thesolution. The high concentration of copper nitrate supplies the necessary copper ions to the electrolytic solution, and the nitrate radical serves to minimize etching. The addition to the bath of a small amount of chromic acid in' amounts up to 2% by weight of the total electrolyte tends to minimize pitting of the article being electropolished. The presence of small amounts of one or more water soluble wetting agents, such as, for example, a sodium salt of an alkyl naphthalene sulphonic acid or an ester of a sulphonated bicarboxylic acid commonly sold under the trade name Aerosol also reduces pitting action. Another strong, stable acid, such as, for example, fluoboric acid, fluosilicic acid, phosphoric acid or a sulfonic acid, may be used in place of the sulphuric acid in an amount which would give the electrolyte substantially the same conductivity as the abovementioned electrolyte.

The latter portion of the bath may be termed the. passivating portion thereof and the preceding portion The temperature of the bath should not be greater than around 110 F. to promote rapid passivation of the surfaceofthe conductor 10'," and should not be below about 65 F. to prevent precipitation of copper sulphate from the bath. The optimum ternperature range is from ab o ut.

755.132 to abouti'90i. The bathfis cooled to a tempera; ture withinQ'this range by suitable cooling un t. (not h wn).

Operation of embodiment shownjn Figs. 1 to 4 The, conductor 10v is advanced continuously through the apparatus, and the cleaning bath. l2 cleansgrease and.- foreign'matter fr'orn'the surface oftheconductor. Thew wateririnsing bath 14,rihses anyalkaliron.theconductor 10. therefrom, and the conductor 10 is .ad.vanced-there-.v

from through. the electropolishingbath. 18; and 'the, cath-. ode device 16 immersed inthe' bath18a The. perforations 4141,in.the cylinder 39; expose only a sufficient area of the sheet .cathode l42 therearound tocause'projections on the surfaceof the conductor to dissolve without passivating the surface .of theconductor,

thecu'r'rentIdensity on the conductor :Withinl the cylinder.

39. being. below 3,000amperes perflsquarevfootb The exposed area of the sheet cathodes 42.42 becomes pro.-.

gressively and uniformly greater from the entrance end.

of .thecylinde'r 39 to a point on the. cylinder 40-.about one-third of its length from the. entranceend thereof, and the'current density on the conductor from .that point to the exit end of the cylinder 40,:a'verages at least13,000.

amperes 'per. square foot.

After the larger projections on the surface of the conductor have been dissolved into the electrolyteinthe projection-dissolving portion of the bath, theincreased current density electropolishes and passivates the surface of the conductor. As the electropolishing proceeds under the high current density, the portion of the. solution immediately surrounding the conductor is broken-downv sufri cientlyl to make oxygen abundantly available. at the .sur-

face of the conductor,and copper oxides build up on the conductor sufiicientlyto. passivate the surfaceIof. the con-w ductor after the surface has received a high ipo'lish. Thepassivating oxidellayer substantiallyrpi'events.further ero-t sion.. The electrolyte is recirculated slowly toprevent agi tation,--and the conductor 10 is' advanced through .thebath 16'sufiiciently. slowly to insure substantially no agita.

tion of the electrolyte around the surface of the conductorlwhich facilitates passivation.

In one specific example, the electropolishing bath :18-

consisted of 19% copper nitrate, 4.5 %sulphuric acid and the balance water, andwas maintained ata temperature of from about,80- F. to about 85- Sulphuric acid was added to the bath-in dilute solution at intervals to the extentnecessary to maintain the correct concentration thereof in th'ebath. In that example, the width of each i of the cathodes 42- -42 was about eightinches, and .thetime of the passage of the conductor through the electrode device 16 wasbetween five' and-ten seconds. "The diameter of the'conductor was 0.1009 1 inch, the spacing between the-cathodes 42-42 and the conductorwas /6 inch and the voltage between the 16volts.' I V The. ratio of thelexposed area of the sheet cathodes to the surface area of the portion of the conductor. coextensive therewith varied gradually fromabout 2m 3 at the entrance end of the cylinder 39 to about 4 to l at the exitend of the projection-dissolving portionof the bath,- and remained at a 4 to l ratio'throughout the passivatin g portion of thebath; In. this example, there was'substan tially no agitation of. the electrolyte aroundthe conductor, and passivation proceeded rapidly. The average current density on the portion of the conductor in the bath 18. was 2,400. amperes per square foot. The conductor isadvanced from the electropolishing bath 18 into and through the rinsing ba th 20 and the alkaline neutralizing bath 22, whichremoves the sur.

conductor and the cathodes was Embodiment shown-. in. Figs.- 5 to 8 This apparatus serves to electropolish-one face-of-a copper tape having serrated-edges 11111*1(Fig. 9). Thetape 110;. is. advanced continuously, by rolls 103108 through a suitable alkaline cleaning bath 112, a water rinsing bath 114, an electropolishing bath 118 having an electrolyte therein of the same composition as that of the electrolyte in the bath 18, a water rinsing bath 120,.analkaline.neutralizingbath 122 and a water rinsing bath 124., The,. tape,1 10, i,drawn from a supply stand (not shown) and is wound ,on a. suitab1e. takeupevi Z Arotarycontact sheave 134contacts the tape 110 at a point priorto theentrance of the tape into .the bath. 118, and "is connected to .the. positive side of a-source.

of :C. potential (not, shown), the negative, side of 1 which is connected ,to a .rotarycontactsheave 140.,(Fig, 7). Feed rolls 142 -1142 advancesa cathode tape 144,

through the bath 118 inthe same general direction as that, in vvhich the, tape, 110 ,is advanced therethrough,land guide rolls 145 -145, guide the. tape. 144in a direction converging toward the tape -11Qfrorn theentrance end of the,electropolishing bath,118,. to. the exit end.there The cathode tape 144 engages the,contact sheave 149, and thereby acquires a negative potential.

As the tape 11Q,,pes ses1througl1 the bath 118, the voltage between the tape 11 0 ,and-thecathode tape 144,

the spaeing betweenthetwo tapes, and the conductivity of thebath are such that any projectionson the. surface of the tape ,110 are ,firstdissolved therefrornby electrolytic action under an anode current density thatis high but below 3,000 amperesper square foot. Theanodev current density isincreased asthe tape 110 approaches the exit end of the cell to. over 3,000 amperesper square foot to form a layer ofcopper oxides on the face of ape-110 nearer th e,ca th ode tap e.,144, and to polish;

and passivate thatface by the time the tape 110 leaves the bath,118.'

'The bath 1 18. is.con tai ned,in a tank 146, supported by .a frame 147; A pair of spaced apertures 148, -148 are provided 'at the entrance end ,of the tank 146-for the passage therethrough fof the tapes 110 and 144. The distance between these ,ta'pes gradually decreases as they approach theexit end of the tank and emerge through a single aperture 149; Amask 150 composedof a nonconductive material, such,as wood or plastic, extends along-the bath 11 8, Iandfcoyers. the back of the tape 110, an d the edges 111- 111 thereof to; prevent dissolving away the serrated edge portions thereof. The

mask 150 includes a back 152 having a groove 154' therein, and this mask is supported.at opposite ends.

of the tank 146 by a pair of braekets 155-155. A scraper lGtL scrape s the cathode tape 144 to remove metal plated on that tape by the electropolishing process. ,Operationpfithe,embodimerrttshown in,Figs. 5 to 8 The tape 110' isadvanced continuously through ;the baths 112, 114; 118',- 1205,1122; and -124M The bath 112 cleans grease and other foreign matter from the tape,'and

the water rinsing bath-114= rinses the alkali from the tape The cathode tape 144 is advanced through the bath 118' along thetape 110' at the same rate of speedas that of the-. tape-110. During the initialportion of the passage of the tape 110 through the bath 118, the resistancebetween the-tapes- 110rand 144 is sufiicientlyhighto prevent rapid passivation of' the fronnface of the-tape-110, but

. v is sufficiently low to cause all larger projections. on the front face of the tape 110 to be electrolytically dissolved rapidly.

The current density on the front face of the tape: 116 increases as it moves from the entrance end of the bath 118 toward the exit end thereof because of the continuous shortening of the path between the two tapes. As the current density increases, the portion of the electrolyte immediately adjacent to the front face of the tape 110 is broken down to provide oxygen, which reacts with the copper coming'from the tape to form oxides on the front face of the tape 110. This face of the tape below the oxides is polished highly by the time the oxides build up to a passivating condition. Slightly before the tape 110 leaves the bath 118, this passivating condition is reached which reduces current flow to a small amount. Thus, the passivation acts as a control to stop substantially further dissolution of copper from the tape 110.

After passivation, the front of the tape 110 has an oxide layer having a lustrous, straw color thereon. This oxide layer is removed from the conductor by the alkaline neutralizing bath 122, which dissolves it and leaves a smooth, golden surface on the front face of the tape.

The movement of the tape 110 through the electropolishing bath 118 is sufficiently slow to prevent agitation around the front face of the portion of the tape in the bath 118. As a result, the breakdown of electrolyte produced along the front face of the tape by the electrolytic action is such that there is sufficient oxygen available at the front face of the tape to form the passivating layer of copper oxides, which limits polishing of this surface when it has become smooth.

in one successful example, the tape 110 was 1% inches in width, each portion of the front face of a tape, except the serrated edge portions, was subjected to electrolytic polishing in an electropolishing bath for a period of from to seconds, and the bath was about four feet long. The cathode tape 144 also was 1 /8 inches in width, the spacing between the tapes 110 and 144 varied from 1% inches at the entrance end of the bath to inch at the exit end thereof, and the potential difference between the tapes was 18 volts. The current density on the face of the tape being polished increased gradually and uniformly from the entrance end of the bath to the exit end thereof, near which the anode current density was over 3,000 amperes per square foot. The average current density on that face of the tape was 1,600 amperes per square foot. The electropolishing bath employed was identical with that used in the previous embodiment of the invention, and it was maintained at about 85 F.

Certain features of the above-described methods and apparatus are disclosed and claimed in my copending application Serial No. 175,125, filed July 21, 1950, for Methods of and Apparatus for Electropolishing Metallic Articles, and in copending application Serial No. 175,189, filed July 21, 1950, by A. N. Gray for Methods of and Apparatus for Electropolishing Metallic Articles.

Copper articles of a nonfilamentary type may be held stationary in the several baths described hereinabove, and stationary cathodes may be used to successfully polish the articles. Such articles may also be advanced continuously through the baths with or without moving electrodes. Such stationary articles may be polished satisfactorily without continuously increasing the current density thereon in the electropolishing bath, while keeping an average anode polishing current density of the order of 3,000 amperes per square foot.

The above-described methods rapidly form lustrous, highly polished surfaces on copper articles originally having minute projections on the surfaces thereof, and the polished surfaces have no pits therein. While the conductor 10 and the tape 110 described hereinabove were made of copper, composite metallic articles having a core composed of one metal, such as, for example, steel and a covering of copper may be successfully electropolished by means of the above-described methods. These methods also may be used in conjunction with suitable electrolytes to successfully electropolish other metals, such as steel, brass, zinc, nickel, stainless steel, etc.

In the use of terms, such as filament and filamentary article, and the like, in the specification and claims, it is intended to include rods, wires, tapes, bands, and the like.

What is claimed is:

l. The method of electropolishing copper articles, which comprises making a copper article an anode in an electrolytic cell including an aqueous bath consisting essentially of from about 18% to about 30% copper nitrate, from about 6% to about 2% sulphuric acid and the balance water, and impressing a D. C. potential upon the cell of suificient magnitude to maintain an average anode current density of at least 3,000 amperes per square foot and to simultaneously release suificient oxygen from the bath around the article to passivate the surface thereof with copper oxides and highly polish the surface of the article beneath the oxides.

2. The method of electropolishing copper filaments, which comprises advancing a copper filament as an anode through an electrolytic cell including an aqueous bath consisting essentially of from about 18% to about 30% copper nitrate, from about 6% to about 2% sulphuric acid and the balance water without agitating the bath materially, impressing a D. C. potential upon the cell of sufiicient magnitude to create an average current density on the filament of at least 3,000 amperes per square foot, maintaining a current density on the portion of the filament in that portion of the bath near the entrance end thereof sufficiently low not to cause passivation of the surface of that portion of the filament but sufficiently high to remove projections from the surface of the filament, and maintaining a current density on the filament between said portion and the exit end of the bath suificiently high to release suflicient oxygen from the bath around the filament to passivate the surface thereof with copper oxides.

3. The method of electropolishing copper articles, which comprises electrolyzing a copper article to be polished as an anode in an electrolyte consisting of copper nitrate about 19% by weight, sulphuric acid about 4.5% by weight and the balance water, maintaining the electrolyte at a temperature of from about F. to about F., impressing a D. C. potential upon the cell to create an anode current density adequate to polish the article, and continuing the electrolysis until the surface of the article is passivated.

4. The method of electropolishing copper articles, which comprises immersing a copper article in an electrolytic cell containing an aqueous electrolyte consisting essentially of from about 18% to about 30% copper nitrate, from about 6% to about 2% sulphuric acid and the balance water, impressing a D. C. potential upon the cell of sufiicient magnitude to create an anode current density on the article adequate to polish the article, and maintaining such potential until eventually sufficient oxygen is released from the electrolyte to passivate the surface of the article by forming an oxide coating thereon.

5. The method of electropolishing copper filaments, which comprises advancing a copper filament as an anode through an electrolytic cell containing an aqueous electrolyte consisting essentially of from about 18% to about 30% copper nitrate, from about 6% to about 2% sulphuric acid and the balance water, impressing a D. C. potential upon the cell of suificient magnitude to create an anode current density on the filament adequate to polish the filament, and gradually increasing the current density until eventually sufficient oxygen is released from the electrolyte to passivate the surface of the filament by forming an oxide coating thereon.

6. The method of electropolishing copper filaments,

which comprises advancing a copper filament as an anode through-an electrolytic-cellcontaining an aqueons electroa lyte=consisting essentially of from about 18% to aboutn 3O-% -copper- :nitrate, from about-6% to about 2% sui'- phuric acidand the-balance 'water, maintainingthe term perature -of the electrolyteat from-about75v F: to about" 90 F., impressing a DT-C; potentiabuponthe'cell of suffi cient magnitude'to create 1 ananodevcurrent density-on the filament adequate to polish the filament, "and gradually increasing the currentidensity sufiicie'ntly to release oxygen. from the electrolyte and eventually tocause the surface of the filament to become= passivated: rapidly -by forming an oxide coating :thereon.

References Cited in the filei of :vthis'patent UNITED STATES PATENTS 1,773,160 Anderson et a1 Aug..19, 1930 8 Fustier Dec. 10, 1935 Manuel's. SeptrZl, 1943 Drumm'ond et alJ a .June :12, 1945' Neill Feb. 20,1951

OTHER REFERENCES Hackhs Chemical Dictionaryf? second edition (1937),

pages 162, .163, definition of -buffer.

Transactions: of The American Electrochemical -So- 

1. THE METHOD OF ELECTROPOLISHING COPPER ARTICLES, WHICH COMPRISES MAKING A COPPER ARTICLE AN ANODE IN AN ELECTROLYTIC CELL INCLUDING AN AQUEOUS BATH CONSISTING ESSENTIALLY OF FROM ABOUT 18% TO ABOUT 30% COPPER NITRATE, FROM ABOUT 6% TO ABOUT 2% SULPHURIC ACID AND THE BALANCE WATER, AND IMPRESSING A D. C. POTENTIAL UPON THE CELL OF SUFFICIENT MAGNITUDE TO MAINTAIN AN AVERAGE ANODE CURRENT DENSITY OF AT LEAST 3,000 AMPERES PER SQUARE FOOT AND TO SIMULTANEOUSLY RELEASE SUFFICIENT OXYGEN FROM THE BATH AROUND THE ARTICLE TO PASSIVATE THE SURFACE THEREOF WITH COPPER OXIDES AND HIGHLY POLISH THE SURFACE OF THE ARTICLE BENEATH THE OXIDES. 